Consensus' is used in several contexts in crypto, in ways drawing from its wider meaning as 'a collective judgment' or either referring to or suggesting analogy to 'blockchain consensus.' 'Blockchain consensus' and 'consensus mechanisms' more narrowly refer to a family of processes for synchronizing network nodes and determining what data is included on a blockchain and which transactions are valid.
Blockchain consensus mechanisms are typically deliberately designed to express certain cryptographic properties, to motivate specific behaviors in network participants, or to be immune to certain types of attacks, with expected functionality supported by mathematical proofs. Distributed processes unrelated to the consensus over database state are sometimes designed with comparable rigor to blockchain consensus processes, and can involve analogous mechanisms for aligning participant behaviors with desired outcome. Oracle, reputation, oversight, adjudicatory, or governance processes, used to reach distributed consensus on truth, quality, intent, correctness, desire, etc are also occasionally referred to as a project's consensus mechanism, with the accompanying arguments that the system will behave as designed or not be subject to certain attacks described as 'proofs.'